1. Field of the Invention
The present invention relates to a photo-sensing device used for monitoring an output of a semiconductor laser or as a photo-sensor of a light communication system.
2. Related Background Art
FIGS. 1A and 1B respectively show a top view and an X--X' sectional view of a structure of a prior art photo-sensing device. As shown, in the prior art photo-sensing device, a semiconductive crystal layer 2 of a lightly doped first conductivity type is formed on a surface of a semiconductor substrate 1 of a heavily doped first conductivity type, having an electrode of the first conductivity type formed on an underside thereof. The semiconductive crystal layer 2 is lamination of a buffer layer 2a, a photo-sensing layer 2b and a window layer 2c, in sequence. Impurities are selectively diffused into the semiconductive crystal layer 2 to form a first region 3 of a second conductivity type. This is a pin photo-diode structure where the semiconductor layer 1 is an n layer (or a p layer), the semiconductive crystal layer 2 is an i layer and the first region 3 is a p layer (or an n layer), and a photo-sensing region 9 is formed therein. An electrode 5 of a second conductivity type is formed on the region 3 on the surface of the semiconductive crystal layer 2, and the region 3 inside the electrode 5 is covered with an anti-reflection film 4 while the semiconductive crystal layer 2 outside the electrode 5 is covered with a device protection film (i.e. passivation film) 8.
When a reverse bias is applied to the semiconductor device thus constructed, a depletion layer is created in a pn junction area in the semiconductive crystal 2. Thus, an electric field is developed in the depletion layer and electrons and holes generated by a light applied to the photo-sensing region 9 are directed to the first conductivity type semiconductor substrate 1 and the second conductivity type region 3 of different conductivity types, respectively, and accelerated thereby. In this manner, a photo-current is taken out and a light signal is detected.
However, in the structure shown in FIGS. 1A and 1B, when the light is directed to the outside of the region 9, the generated carriers reach the pn junction while they are diffused by a density gradient and are taken out as a photo-current. Since the movement of the carriers by the diffusion is slow, a response waveform to a light pulse includes a tail at the end as shown in FIG. 2A. When such a photo-sensing device is used for the light communication, a light emitted from an optical fiber is condensed so that it is directed to the photo-sensing region 9. However, when a portion of light leaks out of the photo-sensing region 9, it leads to the reduction of the response speed of the photo-sensing device by the reason described above. In a high speed photo-sensing device, the area of the photo-sensing region 9 is reduced to reduce a junction capacitance. As a result, a ratio of light directed to the outside of the photo-sensing region 9 increases and a diffused component which has a low response speed increases. This leads to the degradation of the response speed.
When the light emitted from a rear end plane of the semiconductor laser is sensed by the photo-sensing device to feedback-control a drive current for the semiconductor laser in order to keep the light output of the semiconductor laser at a constant level, if the light emitted from the semiconductor laser spreads to the outside of the photo-sensing region 9 of the photo-sensing device, a low response speed component is generated by the diffusion as described above. This adversely affects to the feedback control.